Hygrostat



Sept. 4, 1928.

G. R. LAWRENCE HYGROSTAT Filed Feb.19, 1927 2 Sheets-Sheet l sept. 4,192s. 1,682,915

' G. R. LAWRENCE HYGROSTAT Filed Feb. 19, 1927 2 Sheets-Sheet 2 PatentedSept. 4, 1928i.V

UNITED STATES GEORGE IR. LAWRENCE, OF CHICAGO, ILLINOIS,

HYGRO STAT.

Application sied Februari 19, 1927. serial No. 169,635.

This invention relates to improvements in hygrostats and the like, andto certain improvements and modifications in the liygrostat shown in myapplication, Serial Number 143,663,1i1ed October 23,1926.

The main objects of this invention are to provide a hygrostat especiallyadaptable for relay control, where the currents controlled by thehygrostat switch may be very weak, and the sparking is notobjectionable; to provide a simple form of hygrostat having very fewparts, and no hinged Joints in the mechanism; to provide such a devicein which the circuit is controlled at a predetermined relative humidity,irrespective of the varying temperatures; and to provide means forcompensating for the variations which are shown by the dierences in theincrements of the readings of the wet and dry bulb thermometers anddi'erent ranges of temperatures.

An illustrative embodiment of this in vention is shown in theaccompanying drawings, in which:

Fig. 1 is a front elevation of the hygrostat.

Fig. 2 is a section taken on the line 2 2 of Fig. 1.

Fig. 3 is a section taken on the line 3-3 of Fig. 4.

Fig. 4 is a section taken on line l---4= of Fi 2.

n this invention, two bi-metallic plates or strips, of unequal length,are anchored, at one end, in spaced relation, to a suitable base. Thelonger plate is covered with a suitable moistened wick which maintainsthe plate at the wet bulb temperature, while the other plate is at thedry bulb or air temperature. As is well known, the bi-metallic platesare bent in proportion to the degree of temperature. If the surroundingair were completely saturated, both plates would be deflected similarlyfor any changes in temperature, except for the difference in the lengthsof the plates, but, at ordinary values of relative humidity, the shorterplate is at a higher temperature, and is therefore deiiected a greateramount. A differential movement'is caused by the two plates. Thisdifferential movement, as will be understood, is a function of therelative humidity.

Cooperating contact members are secured to the free ends of the twoplates, and the control circuit is connected through the plates, fromone to the other. The contacts are adjusted to touch when the upper endsof the plates are at a certain distance apart, so that the contact ismade at a predetermlned relative humidity,` then, as the temperaturechanges without a change in the humidity, the plates will movetogether,'

without changing the relation between the upper ends, or'breaking thecontact, but, if a change in the humidity occurs regardless of thechange in temperature, the relation at the contact will be changedaccordingly. Referring to the drawings, the instrument 1s provided withan insulating base 1, to which is anchored the lower ends of thebimetallic plates 2 and 3. This plate 2 is made the longer, for thepurpose which will be more fully explained hereinafter. The hollow wick4 is slipped over the plate 2, and dips into an open reservoir 5. Theside portions of the .reservoir are extended to provide feet which aresecured to the base by means of the screws 6. For supporting the waterbottle B, a frame comprising spaced brackets 7 secured to a verticallypositioned bar 8 for stiffening, is secured to the base 1. The waterbottle B is held, in an inverted position, between the clips 9 in theframe members 7 with the lower open end vof the bottle supported bysuitable brackets 10 in the reservoir 5. The lower end of the wick 4dips into the reservoir, so that the wick is continually saturated.Suitable arms 11 and12 are attached to the upper free ends of the plates2 and 3, respectively. The inner ends of the arms 11 and 12 are bentupwardly and' an adjustable contact screw 13 is provided in the end ofthe arm 12.

The wires 14 and 15, of a suitable controlling circuit, are attached tothe binding posts 16 and 17, respectively, and are mounted in,

an insulating block 18 which is mounted on the base 1. The binding post16 is connected to the plate 2 by means of a wire 19, and the bindingpost 17 is connected to the opposite plate by means of a wire 20.

For enclosing the instrument, a suitable perforated cover 21 isprovided, which may be secured in any suitable manner to the base 1. Asexplained in the application referred to above, the differences in thereadings of the wet bulb thermometer increase slightly for the highertemperatures. If the plates 2 and 3 were made of equal lengths, thecontact would be made at a lower relative humidity for the highertemperatures than it would be for the lower temperatures.

compensation is ,without departing Therefore, by making the wet platelonger, made for this variation and the contact is made at the samerelative'humidity throughout the ordinary ranges of tem eratures'.

ile but one illustrative embodiment of this invention has been shown anddescribed, many alterations and omissions may be had from the spirit ofthe invention, as defined in the following claim:

A hygrostat comprising a base of insulating material, two bi-metallieplates anchored at one end to the base in spaced parallel relation, andmeans for wetting the surface of the one plate, the free end of saidplates arranged to contact when the plates are in a predeterminedrelative position and control an electric circuit, the wetted platebeing slightly longer than the other plate to compense for thedifferences in the movements of the two plates at'different ranges oftemperatures.

In testimony whereof I affix my signature.

GEO'EGE E. LAWRENCE.

